Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net premiums earned	$ 6,040,609	$ 5,744,418	$ 5,226,537
Insurance-Domestic
Segment Reporting Information [Line Items]
Net premiums earned	4,659,359	4,271,933	3,782,416
Insurance-Domestic | Other Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,546,157	1,449,425	1,259,376
Insurance-Domestic | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,269,785	1,126,704	995,047
Insurance-Domestic | Short-tail Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	925,468	875,898	774,809
Insurance-Domestic | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	548,450	526,344	486,759
Insurance-Domestic | Professional Liability
Segment Reporting Information [Line Items]
Net premiums earned	369,499	293,562	266,425
Insurance-International
Segment Reporting Information [Line Items]
Net premiums earned	772,141	802,375	723,151
Insurance-International | Other Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	103,974	92,411	65,528
Insurance-International | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned	93,728	71,997	84,637
Insurance-International | Short-tail Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	373,415	415,123	336,814
Insurance-International | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	125,628	116,369	130,020
Insurance-International | Professional Liability
Segment Reporting Information [Line Items]
Net premiums earned	75,396	106,475	106,152
Reinsurance-Global
Segment Reporting Information [Line Items]
Net premiums earned	609,109	670,110	720,970
Reinsurance-Global | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	421,811	487,264	507,790
Reinsurance-Global | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 187,298	$ 182,846	$ 213,180